This invention relates to piezoelectric crystalline films comprising zinc oxide having a hexagonal crystal structure, and a method for making the same.
In recent years, crystalline thin films of zinc oxide which show piezoelectricity have become of major interest as a piezoelectric material for various transducers. Such piezoelectric crystalline films of zinc oxide may be produced by various methods such as, for example, vacuum deposition methods, epitaxial growth methods, sputtering methods and ion plating methods. Among these methods, the sputtering methods, particularly, a radio-frequency sputtering method has been used very often lately because it has the advantage that the high growth rate of the oriented crystalline film is possible, thus making it possible to mass-produce piezoelectric crystalline films, industrially.
When making a piezoelectric crystalline film of zinc oxide on a substrate surface by the radio-frequency sputtering method, a ceramic of highly pure zinc oxide has been conventionally used as a film material source. However, even when radio-frequency sputtering is effected with such a source, it results in the formation of piezoelectric crystalline film which are poor in the adhesion to the substrate and low in the quality. In addition, it is difficult with such a film material source to make a piezoelectric crystalline film with a c-axis perpendicular to the substrate or film surface. If a piezoelectric crystalline film of zinc oxide has poor adhesion, various disadvantages occur. For example, when manufacturing an acoustic surface wave filter with such a zinc oxide film, it is difficult to form interdigital transducers on the film surface because of the peeling off of the film. Furthermore, the produced acoustic surface wave filter tends to have disconnection of interdigital transducers, and causes a large propagation loss of acoustic surface waves. Also, if the c-axis of the zinc oxide film is inclined with respect to the axis perpendicular to the substrate surface, the electromechanical coupling factor is lowered, thus making it difficult to produce a piezoelectric crystalline film transducer with good conversion efficiency.
Further, piezoelectric crystalline films of highly pure zinc oxide can be used only at high frequencies, but can not be used at low frequencies since resistivity thereof is not sufficiently high. Thus, an applicable frequency range of such a piezoelectric crystalline film is narrow.
The above matters will be evident from the theory on dielectric relaxation angular frequency (.omega..sub.c) which is given by the following equation: ##EQU1## where .sigma.=conductivity of crystalline film [.OMEGA..sup.-1 m.sup.-1 ]
.epsilon..sub.o =permittivity of vacuum [F/m] PA1 .epsilon..sub.ZnO =permittivity of crystalline film PA1 .rho..sub.o =resistivity of crystalline film [.OMEGA..multidot.m] PA1 .epsilon..sub.ZnO =8.5 PA1 .rho..sub.o =10.sup.6 (.OMEGA..multidot.cm)
From the equation (1), it will be seen that the dielectric relaxation angular frequency .omega..sub.c is inversely proportional to the resistivity of the film.
In general, it is recognized that the piezoelectric crystalline film shows piezoelectricity at frequencies where the following relation exists between the dielectric relaxation angular frequency (.omega..sub.c) and an angular frequency (.omega.) at the used frequency: .omega..sub.c &lt;&lt;.omega.. In other words, the piezoelectric crystalline film can be used as a piezoelectric only at frequencies where the angular frequency (.omega.) is sufficiently higher than the dielectric relaxation angular frequency (.omega..sub.c) (normally, .omega.&gt;.omega..sub.c .times.100).
For example, the dielectric relaxation angular frequency for the piezoelectric crystalline film consisting of highly pure zinc oxide (purity: 99.99%) can be found by the equation (1). ##EQU2## where .epsilon..sub.o =8.854.times.10.sup.-12 (F/M)
Since an angular frequency (.omega.) equals 2.pi.f, it is found that the dielectric relaxation frequency f.sub.c is ##EQU3##
Thus, the piezoelectric crystalline films of highly pure zinc oxide can be used at frequencies higher than 100 MHz. In other words, such piezoelectric crystalline films are applicable only at very high frequencies.
As an improved piezoelectric crystalline film of zinc oxide which overcomes the above disadvantages, there has been proposed a zinc oxide film containing copper. Such piezoelectric crystalline films can be used effectively at high frequences, but can not be used at low frequencies since their resistivity is not sufficiently high. Because, such films have a resistivity of about 10.sup.8 to 10.sup.9 .OMEGA..multidot.cm, so that f.sub.c is 10.sup.3 to 10.sup.4 Hz when calculated by the above equation (1). Thus, the applicable frequencies of such films are not less than 100 KHz.
It has now found that these problems can be solved by incorporating vanadium and manganese together with or without copper into a zinc oxide film.
The incorporation of these additive elements can be achieved by the use of a ceramic of zinc oxide containing vanadium and manganese or a ceramic of zinc oxide containing vanadium, manganese and copper, as a film material source for sputtering.
It is therefore an object of the present invention to provide an improved piezoelectric crystalline film of zinc oxide which overcomes the aforesaid disadvantages and can be used as a transducer with good conversion efficiency in a wide range of low to high frequencies. Another object of the present invention is to provide a method for making such improved piezoelectric crystalline films.
According to the present invention, there is provided a piezoelectric crystalline film on a substrate, the film being a crystalline zinc oxide film with a hexagonal crystal structure, and a c-axis substantially perpendicular to the substrate surface, characterized in that said zinc oxide film contains vanadium and manganese together with or without copper.
In the piezoelectric crystalline films of zinc oxide according to the present invention, additive elements, i.e., vanadium, manganese and copper, may be present in any form such as, for example, oxides and compounds thereof. A category of the compounds includes, without being limited to sulphides, sulphates, phosphates, phosphides, and selenates of the respective additive elements.
Since the content of the additive elements has a great influence on the electrical and physical properties of the films, it is preferred to limit the content of the additive elements to the following ranges. The content of vanadium may range from 0.01 to 20.0 atomic percent when converted into the percentage of vanadium atoms. Because, if the content of vanadium is less than 0.01 atomic percent, the film changes for the worse in the quality, and if more than 20.0 atomic percent, the direction of the crystallographic orientation of the film can not be well controlled, resulting in the worse in the orientation of the piezoelectric crystalline film. The content of manganese may range from 0.01 to 20.0 atomic percent when converted into the percentage of manganese atoms. Because, a small amount of manganese less than 0.01 atomic percent results in the change for the worse in the film quality, and does not contribute to the improvement in the resistivity of the film, and a large amount of manganese more than 20.0 atomic percent results in the change for the worse in the orientation of the piezoelectric crystalline film. The content of copper may range from 0.01 to 20.0 atomic percent when converted into the percentage of copper atoms for the same reasons as mentioned above with respect to manganese.
As a material for the substrate on which a piezoelectric crystalline film is formed, there may be used those such as, for example, metal, glass, ceramics, single crystal, resin, rubber and the like.
The piezoelectric crystalline film of the present invention has a c-axis substantially perpendicular to the substrate surface, thus making it possible to produce piezoelectric transducers with a large electromechanical coupling factor, i.e., good conversion efficiency. Further, the piezoelectric cyrstalline films of the present invention have high resistivity, so that the films are applicable in a wide range of low to high frequencies.
Such films can be made by a method comprising sputtering source materials, i.e., zinc oxide, vanadium and manganese with or without copper onto a surface to form a crystalline zinc oxide film, the sputtering being effected by using a film material source consisting essentially of a ceramic of zinc oxide containing vanadium, and manganese together with or without copper.
The piezoelectric crystalline films of the present invention may be made by any sputtering methods such as, for example, radio-frequency sputtering, co-sputtering method.
These and other objects, features and advantages of the present invention will be further apparent from the following description with respect to examples and the accompanying drawing which shows a diagrammatic view of a known radio-frequency sputtering apparatus used for making piezoelectric crystalline films of the present invention.